Electric power tool

ABSTRACT

An electric power tool includes a motor, a speed reducer unit arranged to deliver the rotational power of the motor and provided with gears, a housing arranged to accommodate the motor and the speed reducer unit, and a speed changing unit for changing a gear reduction ratio of the speed reducer unit. The speed changing unit is arranged in such a position as to be operable outside the housing. The speed changing unit includes an operation lever slidingly operable in a speed changing direction when pushed, an operation detector unit for detecting the operation lever to control electric power supplied to the motor, a shift unit for changing the gear reduction ratio of the speed reducer unit in response to sliding movement of the operation lever, and a slide restraint unit for restraining the sliding operation of the operation lever until the operation detector unit detects the operation lever.

FIELD OF THE INVENTION

The present invention relates to an electric power tool, such as a drilldriver, a disc saw or the like, which has a speed changing functionperformed by a speed reduction mechanism.

BACKGROUND OF THE INVENTION

In general, there are known electric power tools that have a speedchanging function with a view to enhance work efficiency (see, e.g.,Japanese Patent Laid-open Publication No. 63-101545).

One example of the electric power tools is shown in FIG. 15. Thiselectric power tool includes a motor 101 as a driving power source, aspeed reducer unit 102 for delivering the rotational power of the motor101 at a reduced speed, a drive unit (not shown) for delivering therotational power of the speed reducer unit 102 to a tip end tool, aresin-made housing 104 provided with a handle portion 104 a and arrangedto contain the motor 101 and the speed reducer unit 102 therein, anoperation lever 105 and a shift unit 105 a, both of which serve as aspeed changing mechanism for changing the gear reduction ratio of thespeed reducer unit 102, the operation lever 105 being arranged in aposition where it can be operated outside the housing 104, a powerswitch 106 installed in the handle portion 104 a for switching on andoff the power supply of the motor 101, and a battery pack 107 engagedwith the housing 104 for supplying electric power to the motor 101.

As shown in FIGS. 16A and 16B, the operation lever 105 is designed toconvert the tool operation state to a low-speed high-torque state in ahigh load condition (when the work load is heavy) but to a high-speedlow-torque state in a low load condition (when the work load is light).This makes it possible for the electric power tool to perform a desiredtightening task depending on the work load, thereby increasing theefficiency of work.

In case the work load varies in the midst of work, the operation lever105 may be operated during the work to change the gear reduction ratio.This may sometimes cause trouble to the electric power tool. Morespecifically, if the gear reduction ratio is changed with the operationlever 105 during the course of work, namely if the gear 102 a of thespeed reducer unit 102 is shifted when in rotation, the mutuallyengageable gears may make contact with each other during their rotationand may be worn or damaged. This may be a cause of trouble in theelectric power tool. The conventional solution to this problem is toincrease the strength of gears, thereby preventing occurrence oftrouble. In this case, however, the gears need to be made of highstrength metal or formed into a big size, which leads to a problem ofhigh cost and increased weight.

SUMMARY OF THE INVENTION

In view of the above, the present invention provides an electric powertool capable of making it impossible to perform a speed changingoperation until the pushing operation of an operation lever is detected,preventing itself from suffering from trouble which would otherwiseoccur due to the wear or damage of gears of a speed reducer unit causedby the speed changing operation performed during the course of work,enjoying enhanced reliability, reducing the strength required in thegears and assuring reduced cost and weight.

The present invention further provides an electric power tool capable ofmaking it possible to easily construct a slide restraint unit throughthe use of an operation lever and a housing, assuring increasedoperability, reliably restraining movement of the operation lever priorto a speed changing operation, preventing an erroneous operation whichwould otherwise occur when the operation lever is inadvertently touched,increasing the detection accuracy without having to use sensors inplural numbers, preventing wear of a detection member while prolongingthe life span thereof, and preventing damage of precision electronicparts such as a sensor or a switch arranged below the operation levereven when a falling impact force or the like is applied to the operationlever.

In accordance with an aspect of the present invention, there is providedan electric power tool including: a motor as a driving power source forgenerating rotational power; a speed reducer unit arranged to deliverthe rotational power of the motor and provided with two or more gears; adriving unit arranged to deliver the rotational power from the speedreducer unit to a tip end tool; a housing arranged to accommodate themotor, the speed reducer unit and the driving unit therein and providedwith a handle portion; and a speed changing unit for changing a gearreduction ratio of the speed reducer unit, the speed changing unitarranged in such a position as to be operable outside the housing,wherein the speed changing unit comprises an operation lever slidinglyoperable in a speed changing direction when pushed, an operationdetector unit for detecting the operation lever to control electricpower supplied to the motor, a shift unit for changing the gearreduction ratio of the speed reducer unit in response to slidingmovement of the operation lever, and a slide restraint unit forrestraining the sliding operation of the operation lever until theoperation detector unit detects the operation lever.

With this configuration, the slide restraint unit restrains the slidingoperation of the operation lever and makes it impossible to perform aspeed changing operation until the pushing operation of the operationlever is detected by the operation detector unit and until the electricpower supplied to the motor is controlled to obtain the revolutionnumber corresponding to the gear reduction ratio. This makes it possibleto prevent the electric power tool from suffering from trouble whichwould otherwise occur due to the wear or damage of gears of the speedreducer unit caused by the speed changing operation performed during thecourse of work.

The slide restraint unit may include a projection portion provided inone of mutually facing surfaces of the operation lever and the housingand a guide portion provided in the other surface, the projectionportion and the guide portion being configured in such a manner as torestrain sliding movement of the operation lever in the speed changingdirection when the push lever is in a non-pushed position but permit thesliding movement of the operation lever in the speed changing directionwhen the push lever is in a pushed position. In this case, it ispossible to easily construct the slide restraint unit using theoperation lever and the housing.

The guide portion may include a slide operation groove extending in thespeed changing direction and a pair of push operation grooves extendingin a pushing direction of the operation lever from the opposite ends ofthe slide operation groove, the slide operation groove and the pushoperation grooves being continuously formed to have a substantiallyU-like shape. In this case, it is possible to simplify the configurationof the guide portion using the substantially U-shaped groove.

The push operation grooves may be inclined at an obtuse angle withrespect to the slide operation groove. In this case, the operation levermoves, when pushed, in the direction inclined at an obtuse angle withrespect to the slide operation groove and not in the directionperpendicular to the slide operation groove. Therefore, the transitionfrom the pushing operation to the sliding operation occurs smoothly,thereby enhancing the operability of the operation lever.

The speed changing unit may further includes a resilient member forbiasing the projection portion against the guide portion in a directionto restrain the movement of the operation lever and a restraintreleasing unit for moving the projection portion to permit the movementof the operation lever when the operation lever is pushed. In this case,use of the resilient body and the restraint releasing unit makes itpossible to bring the operation lever from a movement-restrained stateinto a movement-permitted state in response to the pushing operation ofthe operation lever. This ensures that the transition from the pushingoperation to the speed-changing sliding operation occurs in a smoothermanner.

The operation detector unit may be designed to detect the operationlever when the operation lever is in a generally middle position betweena non-pushed position and a pushed position. In this case, if theoperation lever is not pushed down by a predetermined amount, theoperation detector unit fails to detect the pushing operation of theoperation lever. This makes it possible to prevent an erroneousoperation of the electric power tool which would otherwise occur whenthe operation lever is touched inadvertently.

The operation lever may include an interrupter plate having apredetermined length in the speed changing direction, the operationdetector unit including a sensor for optically detecting the interrupterplate when the operation lever is pushed. In this case, a singleinterrupter plate is sufficient to cover a plurality of pushingpositions of the operation lever, because the interrupter plate extendsin the speed changing direction. This eliminates the need to use sensorsin plural numbers, while assuring reduced cost and weight. Use of thenon-contact sensor assists in preventing wear of the interrupter plateand prolonging the life span thereof.

The operation lever preferably has an operation surface depressedinwards from an outer surface of the housing. In this case, even if afalling impact force or the like is applied to the operation lever, thehousing can first receive the impact force. This is because theoperation surface of the operation lever is depressed. Therefore, it ispossible to prevent damage of precision electronic parts such as asensor or a switch arranged below the operation lever.

With the electric power tool of the present invention, the sliderestraint unit restrains the sliding operation of the operation leverand makes it impossible to perform a speed changing operation until thepushing operation of the operation lever is detected to control theelectric power supplied to the motor. This makes it possible to preventthe electric power tool from suffering from trouble which wouldotherwise occur due to the wear or damage of gears of the speed reducerunit caused by the speed changing operation performed during the courseof work. Furthermore, it is possible to assure enhanced reliability andto reduce the strength required in the gears. Therefore, it becomespossible, for example, to change the material of gears from metal toresin, thereby reducing the cost and weight of the electric power tool.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become apparentfrom the following description of preferred embodiments, given inconjunction with the accompanying drawings, in which:

FIG. 1 is a side section view showing an electric power tool inaccordance with one embodiment of the present invention;

FIG. 2 is an enlarged section view for explaining a speed changingmechanism employed in the electric power tool;

FIG. 3 is an exploded perspective view for explaining the speed changingmechanism employed in the electric power tool;

FIG. 4 is a perspective view showing the speed changing mechanism, withan operation lever removed for clarity;

FIGS. 5A and 5B illustrate a projection portion kept in a non-pushedposition, i.e., in a slide-restrained state, prior to changing the speedof the electric power tool;

FIGS. 5C and 5D illustrate the projection portion moved to a pushedposition and kept in a slide-permitted state prior to changing the speedof the electric power tool;

FIGS. 5E and 5F illustrate the projection portion slidingly operated tofinish the speed changing operation;

FIGS. 5G and 5H illustrate the projection portion spring-biased into anon-pushed position and kept in a slide-restrained state after changingthe speed of the electric power tool;

FIG. 6A is a perspective view corresponding to FIGS. 5A and 5B, whichshows the projection portion kept in a non-pushed position, i.e., in aslide-restrained state, prior to changing the speed of the electricpower tool, FIG. 6B is a section view taken along line A-A in FIG. 6A,FIG. 6C is a section view taken along line B-B in FIG. 6A, and FIG. 6Dis a section view taken along line C-C in FIG. 6B;

FIG. 7A is a perspective view showing the projection portion pushed to agenerally middle position but still kept in a slide-restrained state,FIG. 7B is a section view taken along line D-D in FIG. 7A, FIG. 7C is asection view taken along line E-E in FIG. 7A, and FIG. 7D is a sectionview taken along line F-F in FIG. 7B;

FIG. 8A is a perspective view corresponding to FIGS. 5C and 5D, whichshows the projection portion moved to a pushed position and kept in aslide-permitted state, FIG. 8B is a section view taken along line G-G inFIG. 8A, FIG. 8C is a section view taken along line H-H in FIG. 8A, andFIG. 8D is a section view taken along line I-I in FIG. 8B;

FIG. 9A is a perspective view corresponding to FIGS. 5E and 5F, whichshows the projection portion slidingly operated to finish the speedchanging operation, FIG. 9B is a section view taken along line J-J inFIG. 9A, FIG. 9C is a section view taken along line K-K in FIG. 9A, andFIG. 9D is a section view taken along line L-L in FIG. 9B;

FIGS. 10A through 10H show another example of the guide portion of thespeed changing mechanism;

FIGS. 10A and 10B illustrate the projection portion kept in a non-pushedposition, i.e., in a slide-restrained state, prior to changing the speedof the electric power tool;

FIGS. 10C and 10D illustrate the projection portion moved to a pushedposition and kept in a slide-permitted state prior to changing the speedof the electric power tool;

FIGS. 10E and 10F illustrate the projection portion slidingly operatedto finish the speed changing operation;

FIGS. 10G and 10H illustrate the projection portion spring-biased into anon-pushed position and kept in a slide-restrained state after changingthe speed of the electric power tool;

FIG. 11A is a perspective view showing another example of the sliderestraint unit, and FIG. 11B is a section view taken along line M-M inFIG. 11A;

FIG. 12A is a perspective view showing the slide restraint unit, withthe push lever portion moved from the position shown in FIGS. 11A and11B to a generally middle position, and FIG. 12B is a section view takenalong line N-N in FIG. 12A;

FIG. 13A is a perspective view showing the slide restraint unit, withthe push lever portion moved from the position shown in FIGS. 11A and11B to a pushed position, and FIG. 13B is a section view taken alongline P-P in FIG. 13A;

FIG. 14A is a perspective view showing still another example of theslide restraint unit, and FIG. 14B is a section view taken along lineQ-Q in FIG. 14A;

FIG. 15 is a side section view showing a conventional electric powertool; and

FIGS. 16A and 16B are section views for explaining the conventionalmanner in which the tool operation state is converted from a low-speedhigh-torque state available in a high load condition (when the work loadis heavy) to a high-speed low-torque state available in a low loadcondition (when the work load is light).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be describedwith reference to the accompanying drawings which form a part hereof.

Referring to FIG. 1, the electric power tool 1 of the present embodimentessentially includes a motor 5 as a driving power source, a speedreducer unit 8 arranged to deliver the rotational power of the motor 5and provided with two or more gears 8 a, a driving unit arranged todeliver the rotational power of the speed reducer unit 8 to a tip endtool, a bearing unit for rotatably supporting the driving unit, ahousing 2 arranged to accommodate the motor 5, the speed reducer unit 8,the driving unit and the bearing unit therein and provided with a handleportion 2 a, and a speed changing mechanism 3 for changing the gearreduction ratio of the speed reducer unit 8, the speed changingmechanism 3 being arranged in a position where it can be operatedoutside the housing 2. In FIG. 1, reference numeral 106 designates apower switch for switching on and off the power supply of the motor 5. Abattery pack for supplying electric power to the motor 5 is omitted fromillustration.

The speed changing mechanism 3 is a slide-type operation switch 50 andis divided into an operation lever 4 (an upper layer portion) slidablein a speed changing direction R when in a pushed state and a lower layerportion 15 a as shown in FIG. 3. The speed changing mechanism 3 includesan operation detector unit 6 for detecting the pushed position of theoperation lever 4 and controlling the electric power supplied to themotor 5 so as to rotate the motor 5 at a revolution number correspondingto a gear reduction ratio, a shift unit 105 a (see FIG. 15) for changingthe gear reduction ratio of the speed reducer unit 8 in response to thesliding movement of the operation lever 4, and a slide restraint unit 7for restraining the sliding operation of the operation lever 4 until theoperation detector unit 6 detects the pushed position of the operationlever 4. Reference numeral 15 in the drawings designates a switch base.In the present embodiment, the speed changing direction R coincides withthe axial direction of a rotation shaft of the motor 5.

The operation lever 4 is operated forwards and backwards as shown inFIGS. 2 and 3 and includes a slide lever portion 4 b slidable only inthe speed changing direction R and a push lever portion 4 a that can bepushed downwards relative to the slide lever portion 4 b. When the slidelever portion 4 b and the push lever portion 4 a are slidingly operatedby pressing the operation surfaces 4 c with a finger, only the pushlever portion 4 a is pushed downwards. As a result, a stepped portion 17(see FIG. 5C and 7B) for making it easy to slide the slide lever portion4 b appear at the border between the operation surfaces 4 c. The pushlever portion 4 a is biased upwards by a switch spring 18. When notpushed, the operation surfaces 4 c of the operation lever 4, includingthe slide lever portion 4 b and the push lever portion 4 a, are all keptflush. In FIG. 3, reference numeral 19 designates a guide shaft andreference numeral 60 designates a switch spring guide.

An interrupter plate 6 a serving as a detection plate is installed toprotrude downwards from the lower end of the push lever portion 4 a. Theinterrupter plate 6 a extends a predetermined length along the speedchanging direction R and has, e.g., opening portions and non-openingportions (not shown) alternately arranged along the longitudinaldirection thereof (i.e., the speed changing direction R). In the presentembodiment, the operation surfaces 4 c of the operation lever 4 aredepressed a predetermined depth W (see FIG. 2) from the outer surface ofthe housing 2.

Below the lower layer portion 15 a of the operation lever 4, a sensorstand 16 for holding a photo interrupter 6 b of the operation detectorunit 6 is attached to the switch base 15. The operation detector unit 6detects the interrupter plate 6 a moved down together with the pushlever portion 4 a when the latter is pushed. Using the detectionresults, the operation detector unit 6 controls the motor 5 in thebelow-mentioned manner so that the motor 5 can rotate at a revolutionnumber corresponding to the gear reduction ratio.

The slide restraint unit 7 restrains the operation lever 4 fromperforming the speed changing operation until the pushing operation ofthe push lever portion 4 a is detected by the photo interrupter 6 b. Asshown in FIG. 3, the slide restraint unit 7 of the present embodimentincludes a pair of projection portions 7 a provided to the push leverportion 4 a and a pair of guide portions 7 b provided on the slidingsurfaces of the housing 2 along which the operation lever 4 makessliding movement. The guide portions 7 b are configured to guide theprojection portions 7 a in such a manner that they restrain the slidingmovement of the projection portions 7 a in the speed changing directionR when the push lever portion 4 a is in a non-pushed position T butpermits the sliding movement of the projection portions 7 a in the speedchanging direction R when the push lever portion 4 a is pushed. As shownin FIGS. 4 and 5A through 5H, each of the guide portions 7 b includes,for example, a slide operation groove 10 extending in the speed changingdirection R and a pair of push operation grooves 9 extending in apushing direction S of the operation lever 4 from the opposite ends ofthe slide operation groove 10. The slide operation groove 10 and thepush operation grooves are continuously formed to have a substantiallyU-like shape.

Next, description will be made on the operation of the electric powertool.

In order to change the speed of the electric power tool 1, a user slidesthe operation lever 4 while pushing the same with a finger. In thisregard, FIGS. 5A and 5B illustrate the projection portion 7 a kept in aslide-restrained state prior to changing the speed of the electric powertool 1. FIGS. 5C and 5D illustrate the projection portion 7 a kept in aslide-permitted state. FIGS. 5E and 5F illustrate the projection portion7 a slidingly operated to finish the speed changing operation. FIGS. 5Gand 5H illustrate the projection portion 7 a spring-biased into thenon-pushed position T and kept in the slide-restrained state afterchanging the speed of the electric power tool 1. FIGS. 6A through 6Dillustrate the positional relationship between the interrupter plate 6 aand the photo interrupter 6 b before the speed changing operation (orafter the speed changing operation), which views correspond to FIGS. 5Aand 5B (or FIGS. 5G and 5H). In FIGS. 6A through 6D, reference letter“T” indicates the non-pushed position, “T1” indicates the generallymiddle position where the interrupter plate 6 a is detectable by thephoto interrupter 6 b, “P1” indicates the push-in amount up to T1, “T2”indicates the pushed position where the sliding movement is permitted,and “P2” indicates the push-in amount up to T2. FIGS. 7A through 7Dillustrate a state in which the push lever portion 4 a is pushed in upto the generally middle position T1 where the interrupter plate 6 a isdetectable by the photo interrupter 6 b. FIGS. 8A through 8D illustratea state in which the push lever portion 4 a is pushed into a positionwhere the sliding movement is permitted. FIGS. 9A through 9D illustratethe positional relationship between the interrupter plate 6 a and thephoto interrupter 6 b after the speed changing operation, which viewscorrespond to FIGS. 5E and 5F.

If the push lever portion 4 a of the operation lever 4 is pushed asshown in FIGS. 5A and 5B, the projection portion 7 a is moved down alongthe push operation groove 9. The movement of the projection portion 7 ainto the slide operation groove 10 is restrained when the push leverportion 4 a is in the generally middle position T1. This makes itimpossible to change the speed of the electric power tool 1. In thegenerally middle position T1, the interrupter plate 6 a is detected bythe photo interrupter 6 b. For example, by sensing one of the openingportions and non-opening portions of the interrupter plate 6 a, thephoto interrupter 6 b detects whether the operation lever 4 is in ahigh-speed state or a low-speed state. Using this detection result, acontrol unit (not shown) controls the electric power supplied to themotor 5. When the high-speed state is detected, the motor 5 is convertedfrom high speed rotation to low speed rotation. In contrast, when thelow-speed state is detected, the motor 5 is converted from low speedrotation to high speed rotation. After the push lever portion 4 a ispushed into the pushed position T2 to permit sliding movement, theoperation lever 4 including the push lever portion 4 a and the slidelever portion 4 b is slidingly operated to perform the speed changingoperation. When performing the speed changing operation, the motor 5 isalready driven at a revolution number corresponding to the gearreduction ratio as mentioned above. Therefore, it is possible to preventthe gears of the speed reducer unit 8 from being worn or damaged by themutual collision during their rotation, thereby avoiding occurrence ofproblems or trouble which would otherwise be caused by the speedchanging operation performed during the course of work.

With the configuration stated above, the slide restraint unit 7restrains the sliding movement of the operation lever 4 and makes itimpossible to perform the speed changing operation until the pushingoperation of the push lever portion 4 a of the operation lever 4 isdetected by the operation detector unit 6. As a result, the operationdetector unit 6 performs its detection task in a reliable manner and theelectric power supplied to the motor 5 is controlled so that the motor 5can rotate at the revolution number corresponding to the gear reductionratio. Therefore, it becomes possible to prevent the electric power toolfrom suffering from trouble which would otherwise occur due to the wearor damage of the gears 8 a of the speed reducer unit 8 caused by thespeed changing operation performed during the course of work.Furthermore, it is possible to assure enhanced reliability and to reducethe strength required in the gears 8 a of the speed reducer unit 8.Therefore, it becomes possible, for example, to change the material ofthe gears 8 a from metal to resin. This eliminates the need to make thegears 8 a from high strength metal or to increase the size of the gears8 a, eventually making it possible to avoid an increase in the cost andweight of the electric power tool 1.

The photo interrupter 6 b detects the push lever portion 4 a when thelatter is in the generally middle position T1. In other words, the photointerrupter 6 b does not detect the push lever portion 4 a unless thelatter is pushed down by a predetermined amount. This makes it possibleto prevent an erroneous operation of the electric power tool which wouldotherwise occur when the push lever portion 4 a is touchedinadvertently. Owing to the fact that the interrupter plate 6 a extendsin the speed changing direction R, a single interrupter plate issufficient to cover a plurality of pushing positions T2 of the pushlever portion 4 a. This eliminates the need to use a sensor, e.g., thephoto interrupter 6 b, in plural numbers, while assuring reduced costand weight. Use of the non-contact sensor assists in preventing wear ofthe interrupter plate 6 a and prolonging the life span thereof. Sincethe photo interrupter 6 b is a non-contact sensor, it can be used for along period of time. In addition, the lead wire through which to send adetection signal from the sensor to a power supply circuit of the motor5 is kept stationary regardless of the operation of the operation lever4. This reduces the probability that the lead wire is flexed andeventually disconnected, thereby making it possible to increasereliability.

The slide restraint unit 7 of the present embodiment includes theprojection portions 7 a provided to the push lever portion 4 a of theoperation lever 4 and the guide portions 7 b provided in the housing 2.This makes it possible to easily construct slide restraint unit 7 byusing the operation lever 4 and the housing 2. Furthermore, each of theguide portion 7 b includes the slide operation groove 10 extending inthe speed changing direction R and the pair of push operation grooves 9extending in the pushing direction S from the opposite ends of the slideoperation groove 10. The slide operation groove 10 and the pushoperation grooves are continuously formed to have a substantially U-likeshape. This makes it possible simplify the configuration of the guideportion 7 b. In addition, since the guide portions 7 b are provided inthe housing 2 and the projection portions 7 a are provided to theoperation lever 4, it is possible to reduce the size of the slide-typeoperation switch 50.

There may be a fear that the precision electronic parts (e.g., thesensor such as the photo interrupter 6 b or the like and the switch suchas the operation detector unit 6 or the like) arranged just below theoperation lever 4 are damaged if a falling impact force or the like isapplied to the operation lever 4. In the present embodiment, theoperation surfaces 4 c of the operation lever 4 are depressed by apredetermined depth W (see FIG. 2). Therefore, the housing 2 can firstreceive the impact force. This makes it possible to prevent damage ofthe sensor.

FIGS. 10A through 10H show another example of the substantially U-shapedgrooves of the guide portion 7 b. In this example, a pair of pushoperation grooves 9 is inclined at an obtuse angle θ with respect to aslide operation groove 10. The remaining structures are the same asthose of the embodiment shown in FIGS. 1 through 3. In this example, thepush operation grooves 9 extend continuously from the slide operationgroove 10 in an upwardly diverging shape. As a result, when the pushlever portion 4 a is pushed, it does not move down vertically but movesobliquely toward the slide operation groove 10. Therefore, thetransition from the pushing operation to the sliding operation occurssmoothly, thereby enhancing the operability of the operation lever 4.

FIGS. 11A, 11B, 12A, 12B, 13A and 13B show another example of the guideportion 7 b. In this example, there are provided resilient bodies 12 forbiasing the projection portions 7 a in a movement-restraining directionrelative to the guide portions 7 b and restraint releasing units 13 forbiasing the projection portions 7 a in a movement-permitting directionrelative to the guide portions 7 b when the operation lever 4 is pushed.The remaining structures are the same as those of the embodiment shownin FIGS. 1 through 3. In this example, a pair of left and rightprojection portions 7 a is arranged on the opposite sides of the sensorstand 16 as shown in FIG. 11B. The projection portions 7 a have the samestructure. Coil springs as the resilient bodies 12 protrude from theinner ends of the projection portions 7 a. The sensor stand 16 hasspring rests 70 arranged to support the tip ends of the coil springs.Triangular lug portions protrude upwards from the inner upper surfacesof the projection portions 7 a. Each of the lug portions has an outertapering surface 13 a. Restraint releasing arms 13 b extend downwardsfrom the lower opposite side surfaces of the push lever portion 4 a. Therestraint releasing arms 13 b and the tapering surfaces 13 a of the lugportions constitute the restraint releasing units 13.

When the operation lever 4 of this example is in the non-pushed positionT, the projection portions 7 a are resiliently pressed against the guideportions 7 b by the coil springs as shown in FIG. 11B, thus restrainingthe sliding movement of the operation lever 4. If the push lever portion4 a of the operation lever 4 is pushed, the restraint releasing arms 13b are slidingly moved down over the tapering surfaces 13 a of theprojection portions 7 a. Thus the projection portions 7 a move away fromthe guide portions 7 b. If the push lever portion 4 a reaches thegenerally middle position T1 as shown in FIG. 12B, the interrupter plate6 a is detected by the photo interrupter 6 b. When the push leverportion 4 a is further pushed into the pushed position T2 as shown inFIG. 13B, the sliding movement of the projection portions 7 a relativeto the guide portions 7 b is permitted so that the speed changingoperation can be performed by slidingly operating the operation lever 4.As set forth above, the slide restraint unit 7 of this example iscapable of bringing the projection portions 7 a from amovement-restrained state into a movement-permitted state in response tothe pushing operation of the push lever portion 4 a of the operationlever 4. This ensures that the transition from the pushing operation tothe speed-changing sliding operation occurs in a smoother manner.Another advantage resides in that it is possible to easily construct theslide restraint unit 7 using the coil spring-biased projection portions7 a provided in the operation lever 4 and the guide portions 7 bprovided in the housing 2.

FIGS. 14A and 14B show an example in which the guide portions 7 binclude grooves cut in the radial direction (i.e., the thicknessdirection) Y of the housing 2. As is the case in FIGS. 4 and 6A through6D, these grooves have a substantially U-like shape when seen from theinside of the housing 2 and are opened downwards. The remainingstructures are the same as those of the embodiment shown in FIGS. 1through 3. In this example, projection portions 7 a protrude from theleft and right end regions of the push lever portion 4 a. Each of theprojection portions 7 a are formed into a generally L-like shape. Thetip ends of the projection portions 7 a are inserted into thedownwardly-opened guide portions 7 b of the housing 2. The sensor stand16 includes spring rests 70 provided at the left and right sidesthereof. Coil springs as resilient bodies 12 for biasing the projectionportions 7 a in a movement-restraining direction with respect to theguide portions 7 b are retained between the spring rests 70 and thelower surface of the push lever portion 4 a. When the operation lever 4of this example is in the non-pushed position T, the projection portions7 a are resiliently pressed against the guide portions 7 b by the coilsprings as shown in FIG. 14B, thus restraining the sliding movement ofthe operation lever 4. If the push lever portion 4 a of the operationlever 4 is pushed, the coil springs are compressed and the tip ends ofthe projection portions 7 a are moved away from the guide portions 7 b.When the push lever portion 4 a is in the generally middle position T1,the interrupter plate 6 a is detected by the photo interrupter 6 b. Ifthe push lever portion 4 a reaches the pushed position T2, the slidingmovement of the projection portions 7 a relative to the guide portions 7b is permitted so that the speed changing operation can be performed byslidingly operating the operation lever 4.

As set forth above, the slide restraint unit 7 of this example iscapable of bringing the projection portions 7 a from amovement-restrained state into a movement-permitted state in response tothe pushing operation of the push lever portion 4 a of the operationlever 4. This ensures that the transition from the pushing operation tothe speed-changing sliding operation occurs in a smoother manner.Furthermore, it is possible to easily construct the slide restraint unit7 using the projection portions 7 a and the resilient bodies 12 providedto the operation lever 4 and the guide portions 7 b provided in thehousing 2. Owing to the fact that the guide portions 7 b are formed toextend in the radial direction (i.e., the thickness direction), itbecomes easy to reduce the circumferential size of the housing 2. Sincethe guide portions 7 b are opened downwards, it is possible to preventdust from gathering in the guide portions 7 b.

Although the operation lever 4 is divided into the slide lever portion 4b and the push lever portion 4 a and only the push lever portion 4 a ispushed according to the foregoing embodiment, the present invention isnot limited thereto. Alternatively, the operation lever 4 may be formedinto a single piece so that the sliding operation can be performed whilepushing the operation lever 4 as a whole.

Although the photo interrupter 6 b is used as the operation detectorunit 6 and the interrupter plate 6 a is used as the detected plateaccording to the foregoing embodiment, other sensors such as a magneticsensor and the like may be used instead of the combination of the photointerrupter 6 b and the interrupter plate 6 a. As a further alternative,it may be possible to use a typical mechanical contact switch, e.g., atact switch, a limit switch or a micro switch.

Although the speed changing direction R is the back-and-forth directionparallel to the axial direction D of the rotation shaft of the motor 5according to the foregoing embodiment, the present invention is notlimited thereto. As an alternative example, the speed changing directionR may be the left-and-right direction perpendicular to the rotationshaft of the motor 5. In this case, the guide portion 7 b may be asubstantially U-shaped groove extending in the circumferential directionof the housing 2. This assists in reducing the radial size of thehousing 2.

While the invention has been shown and described with respect to thepreferred embodiments, it will be understood by those skilled in the artthat various changes and modification may be made without departing fromthe scope of the invention as defined in the following claims.

1. An electric power tool comprising: a motor as a driving power sourcefor generating rotational power; a speed reducer unit arranged todeliver the rotational power of the motor and provided with two or moregears; a driving unit arranged to deliver the rotational power from thespeed reducer unit to a tip end tool; a housing arranged to accommodatethe motor, the speed reducer unit and the driving unit therein andprovided with a handle portion; and a speed changing unit for changing agear reduction ratio of the speed reducer unit, the speed changing unitarranged in such a position as to be operable outside the housing,wherein the speed changing unit comprises an operation lever slidinglyoperable in a speed changing direction when pushed, an operationdetector unit for detecting the operation lever to control electricpower supplied to the motor, a shift unit for changing the gearreduction ratio of the speed reducer unit in response to slidingmovement of the operation lever, and a slide restraint unit forrestraining the sliding operation of the operation lever until theoperation detector unit detects the operation lever.
 2. The electricpower tool of claim 1, wherein the slide restraint unit includes aprojection portion provided in one of mutually facing surfaces of theoperation lever and the housing and a guide portion provided in theother surface, the projection portion and the guide portion beingconfigured in such a manner as to restrain sliding movement of theoperation lever in the speed changing direction when the push lever isin a non-pushed position but permit the sliding movement of theoperation lever in the speed changing direction when the push lever isin a pushed position.
 3. The electric power tool of claim 2, wherein theguide portion includes a slide operation groove extending in the speedchanging direction and a pair of push operation grooves extending in apushing direction of the operation lever from the opposite ends of theslide operation groove, the slide operation groove and the pushoperation grooves being continuously formed to have a generally squarebracket shape.
 4. The electric power tool of claim 3, wherein the pushoperation grooves are inclined at an obtuse angle with respect to theslide operation groove.
 5. The electric power tool of claim 2, whereinthe speed changing unit further comprises a resilient member for biasingthe projection portion against the guide portion in a direction torestrain the movement of the operation lever and a restraint releasingunit for moving the projection portion to permit the movement of theoperation lever when the operation lever is pushed.
 6. The electricpower tool of claim 2, wherein the projection portion is provided to theoperation lever and the guide portion is provided to the housing.
 7. Theelectric power tool of claim 1, wherein the operation detector unit isdesigned to detect the operation lever when the operation lever is in agenerally middle position between a non-pushed position and a pushedposition.
 8. The electric power tool of claim 1, wherein the operationlever includes an interrupter plate having a predetermined length in thespeed changing direction, the operation detector unit including a sensorfor optically detecting the interrupter plate when the operation leveris pushed.
 9. The electric power tool of claim 1, wherein the operationlever has an operation surface depressed inwards from an outer surfaceof the housing.